The present invention relates to a magneto-optical recording medium which is capable of performing over-write recording.
In recent years, considerable attention has been concentrated on magneto-optical recording materials for recording information therein by writing magnetic domains on a thin magnetic film thereof, utilizing the thermal effect of light, and for reading out the recorded information, therefrom by utilizing the magneto-optical effect thereof.
In the conventional magneto-optical recording, a laser beam modulation method is used in which a laser beam emitted from a semiconductor laser by inputting an information signal thereto is applied to a magneto-optical recording medium, and at the same time, a magnetic field is applied to the recording medium in the recording direction thereof, whereby thermal magnetic recording is performed. When the recorded information is deleted in this laser beam modulation method, a magnetic field for deletion (hereinafter referred to as "deleting magnetic field") is applied to the recording medium in the direction opposite to the direction at the time of recording, and at the same time, a laser beam is applied to the recording medium. This process will now be explained with reference to FIG. 1A. In a magneto-optical recording medium 33 which comprises a magnetic layer 13 shown in the figure, information A and information B (not shown) are recorded in the magnetic layer 13 with an area a of the magnetic layer 13 corresponding to information A and an area b of the magnetic layer 13 corresponding to information B being magnetized in the opposite direction. When information C and information d (not shown) are newly recorded in the magnetic layer 13, a recording magnetic field Hw is applied to the magnetic layer 13 in the recording direction thereof as shown in FIG. 1B, and at the same time, a laser beam 31 is applied pulse-wise to an area c of the magnetic layer 13 corresponding to information C and an area d of the magnetic layer 13 corresponding to information D of the magnetic layer 13. By this process, the magnetized directions of the areas c and d are reversed so that information C and information D are recorded.
However, information A and information C cannot be deleted simultaneously during the above-mentioned process. In order to delete information A and information B, it is necessary to heat the areas a and b by application of a laser beam thereto, and to apply a deleting magnetic field to the areas a and b in the direction opposite to the recording magnetic field Hw. However, in order to input information to the recording medium, with distinction between an information recording signal and an information deleting signal, it will be necessary to recognize the previously recorded bits and reverse the direction of the magnetic field in synchronization with the deleting signal. However, it will be extremely difficult to perform this in practice.
On the other hand, in a conventional recording method in which information is recorded in a magnetic recording medium by a magnetic head, an electric current for generating a magnetic field in a deleting direction is always input to the magnetic head, and an electric current for generating a magnetic field in an opposite direction is input to the magnetic head when recording information. Therefore, even if some information is already recorded in the recording medium, it is deleted whenever another information is newly recorded in the magnetic recording medium.
Thus, in the conventional magneto-optical recording method, over-write recording, that is, writing new information over already recorded information, is impossible, unlike the conventional magnetic recording. This is the shortcoming of the conventional magneto-optical recording method.
The inventors of the present invention previously proposed a magnetoplumbite hexagonal ferrite as a magnetic material for a magneto-optical recording medium. For example, in Japanese Laid-Open patent application No. 59-45644, a magnetoplumbite hexagonal ferrite with improved magneto-optical characteristics was proposed, which is represented by the following formula (I), in which part of Fe atoms is replaced by metal atoms other than Fe atoms: EQU AO.n[M.sub.x Fe.sub.2-x O.sub.3 ] (I)
wherein A represents Ba, Sr, La or Ca; 5.ltorsim.n.ltorsim.6; and M represents In, Sc, Ga, Rh, Cr, C, Co, Ni, Ti, Zn or Sn.
Since magnetoplumbite hexagonal ferrite itself is an oxide, it is neither oxidized nor corroded, and therefore is stable. Further, it has excellent magneto-optical characteristics. For instance, its Faraday effect can be used since it is highly transparent. However, magneto-plumbite hexagonal ferrite cannot readily be used as it is in a magneto-optical recording medium for over-write recording.